This invention relates to a so-called metallic oxide-hydrogen battery in which a metallic oxide and hydrogen are employed as a positive electrode active material and a negative electrode active material, respectively, and more particularly it relates to a hermetically sealed metallic oxide-hydrogen battery in which a hydrogen negative electrode is composed of a novel composition of a hydrogen storage alloy, its internal pressure is maintained at a lower level, and its self-discharge is also controlled to keep up a long lifetime.
At present, much attention has been paid to the metallic oxide-hydrogen battery in which the hydrogen negative electrode is consitituted of the hydrogen storage alloy. The reason is that such a battery system is capable of a safe operation because it can greatly lower the hydrogen pressure in the battery as compared with a conventional metallic oxide-hydrogen battery which does not employ hydrogen storage alloy, and has a capability of being more great a battery capacity per volume.
The hydrogen storage alloy which has often been heretofore studied as the hydrogen negative electrode for this type of battery is LaNi.sub.5 (for example, U.S. Pat. No. 3,874,928). Further, an alloy of Ni and a mischmetal (hereinafter referred to as Mm) which is a mixture of lanthanum elements such as La, Ce, Pr, Nd and Sm, that is, an MmNi.sub.5 has been also studied.
In the case that such a hydrogen storage alloy is employed in a metallic oxide-hydrogen battery, the internal pressure due to hydrogen of the battery is indeed lower than that of the battery (50 kg/cm.sup.2 or less) in which any hydrogen storage alloy is not used. However, the internal pressure of the battery including the hydrogen storage alloy is still within the range of 2 to 5 kg/cm.sup.2 at ordinary temperature since the equilibrium plateau pressure of these hydrogen storage alloys is not sufficiently low.
When the hydrogen pressure in the battery is higher than atmospheric pressure, a battery container must be structurally strongly manufactured to some extent, and further the following disadvantageous problems will be characteristically induced: A first problem is that since hydrogen molecules are small in molecular diameter, they will inevitably leak from the battery container, though the leakage is gradual, which will noticeably impair its safety. And a second problem is that the metallic oxide electrode as the positive electrode is reduced by the hydrogen gas in the battery so that its battery capacity will decrease and thereby self-discharge of the battery will be led.
For these reasons, it has been suggested to employ, as the hydrogen negative electrode, a hydrogen storage alloy having a low equilibrium plateau pressure, and there have been conducted researches on such a kind of various alloys.
For example, with regard to LaNi.sub.5 and MmNi.sub.5, their equilibrium plateau pressures are as high as about 3 atm and 15 atm, respectively at ordinary temperature, but if a portion of Ni therein is replaced with another element, their equilibrium plateau pressures will be lowered. Especially, a ternary alloy in which a portion of Ni is replaced with manganese (Mn) has been considered to be most preferable as the material for the hydrogen negative electrode, because it will scarcely induce the decrease in an amount of storaged hydrogen, that is, the deterioration in an electrode capacity when compared with other alloys in which a portion of Ni is replaced with elements other than Mn (for example, see "Hydrogen Electrochemical Storage by Substituted LaNi.sub.5 Compound", A. Percheron-Guegan et al. in Hydrides for Energy Storage at page 485 (A. F. Andresen et al. eds. 1978), published by Pergamon Press).
However, if the hydrogen negative electrode is actually made from the above ternary hydrogen storage alloy including Mn and is repeatedly subjected to a charge/discharge cycle in an aqueous solution of an alkaline such as KOH or NaOH, a lifetime of the negative electrode will expire when the charge/discharge cycle has been repeated 50 to 100 times.
On the other hand, in order to hermetically seal a battery, it is a problem which should be considered that the prevention of increment in the inner pressure of a battery due to oxygen generated from a positive electrode at the last stage of charging.
In the hermetically sealed alkaline storage battery, the negative electrode is generally designed so as to be greater in capacity than the positive electrode, and a part of an excess amount being in the discharge state and the remainder in the charge state. The reason for this is to achieve a rapid absorbing (or reducing), on the negative electrode, of the generating oxygen gas from the positive electrode at the last stage of charging and overcharged state. The internal pressure of the battery can be controlled at a low level and the battery can be maintained in the hermetical condition so long as the above-mentioned absorption reaction makes smooth progress.
Since it is proportional to the oxygen pressure, a rate of the oxygen absorption reaction will be accelerated along with a rise of the oxygen pressure in the battery, and the reaction rate will subsequently be equal to an oxygen generating rate (charging current) at a certain pressure. At this point of time, the rise of the battery internal pressure will be finally halted and show a constant value, but until this equilibrium point, on the negative electrode, the charge reaction rate (charging current) of the active material will be greater than the rate of the oxygen absorption reaction. Therefore, the capacity which is to be discharge state in the excess capacity of the negative electrode must be excessively greater by at least an electrical capacity content required in the interval of to a point that the internal pressure shows a constant value, i.e. a point that the quantitative absorption begins.
In the practical use of the hermetically sealed alkaline storage battery employing the hydrogen storage alloy electrode, there is one serious problem that the absorbing reaction of the above oxygen gas (the oxygen reducing reaction) on the hydrogen storage alloy negative electrode is slow.
This oxygen absorbing rate being slow means that a high oxygen pressure is necessasry until the quantitative absorption of oxygen begins, so that the capacity of the negative electrode which should be in excess of the positive electrode must be increased as much as a corresponding electrical capacity. In addition, the oxygen internal pressure of the battery will increase, whereby expansion of the battery and leakage of the liquid will be apt to occur. Further, the negative electrode will occupy a large space in the battery container having a fixed size, therefore the capacity of the battery will be decreased and the effect of using the hydrogen storage alloy electrode will be reduced.
Accordingly, an attempt is made to incorporate an oxyen reducing catalyst into the hydrogen storage alloy electrode for the purpose of accelerating the oxygen absorbing rate on the negative electrode. As the most effective one of such oxygen reducing catalysts, platinum or silver is known. However, platinum or silver is expensive, and it has additionally been found that if platinum or silver is incorporated into the hydrogen storage alloy electrode, its oxygen absorbing power is not so high than the capacity to be expected and the internal pressure of the battery will rise.
One of the reasons that the oxygen absorbing rate is not so fast on the hydrogen storage alloy negative electrode is the way the electrode is prepared. The hydrogen storage alloy will be finely pulverized at the time of the absorption of hydrogen. Therefore, the application of the hydrogen storage alloy as the hydrogen electrode to the negative electrode of the battery is often accomplished by mixing the previously powdered alloy with a plastic binder, bringing the mixture to a paste-like or a sheet-like kneaded material, and causing the material to compressedly and integrally adhere to a current collector in order to thereby form a so-called plastic-bonded electrode. This system can be prepared more easily than a sintered electrode but has the following drawbacks.
Namely, the plastic-bonded negative electrode has a smaller surface porosity and area which actually contacts with oxygen will become low as compared with the sintered system negative electrode. Thus, the oxygen absorbing rate of the plastic-bonded negative electrode will be lower.
The plastic binder system electrode has also the problem of a mechanical peeling. In this system, the kneaded sheets material is, in general, compressedly adhered to the opposite sides of the current collector to form an integral body, but when the material is wound spirally, the outside kneaded sheets material will tend to peel off on the opposite sides of the current collector because of their different curvatures. This phenomenon will substantially lead to the reduction in the capacity of the electrode and will cause the ceterioration in the battery properties.
In the aforesaid "Hydrogen Electrochemical Storage by Substituted LaNi.sub.5 compound", A. Percheron-Guegan et al., there is described a LaNi.sub.5-x Al.sub.x and LaNi.sub.5-x Mn.sub.x alloy, but there is no description concerning an electrode using a quaternary alloy comprising La-Ni-Mn-Al, and no reference is made to to life span. Further, in "Hydrogen absorption-Desorption Characteristics of Mischmetal-Ni-Aluminum alloys in Hydrogen atmosphere", Y. Osumi et al., J. Less-Common Metals, 66, 67 (1979), there is a description concerning Mm-Ni-Al alloy, but it is not used for an electrode.